my guess: MC's a dunce who's been lying to her about something completely innocuous, like "oh we met on this day X years ago so this is our anniversary, I was hiding your present from you" and because this is a romcom manga, he's hit super hard by the fact that her brother knew + exposed him as a liar. She'll confront him asking him to come clean about his lies, he'll tell her, and then everyone else will feel like an idiot for doubting the MC while the MC remains oblivious to all the drama he caused.
It probably has nothing to do with the backstory the brother introduced, and it'll all go back to the status quo again for a while.